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The Radium King · 04-29-2016, 06:59 AM

yessir. as has been said, without the maf the ecu uses the throttle position sensor to estimate how much fuel to provide. called alpha N tuning i recall, and is used by some race companies (gets the maf out of the system for better airflow). in our applications it is a limp mode, however, so, as stated, the ecu adds additional fuel to keep things on the rich side (rich burns fuel and kills power, lean kills engines).

the guys who put the 3.4 in your car probably just kept the old tune on it - the ecu is has enough range to adapt to the increased airflow required for the bigger engine. however, note that the ecu calculates airflow by looking at the maf signal and multiplying it by the diameter of the maf holder. increase the maf holder diameter without telling the ecu and it starts doing bad math - there is more air getting to the engine than the ecu knows about and the engine runs lean. your engine will run like crap.

your options are to:

(a) go back to the original, smaller maf holder.
(b) push an oem tune onto the car that knows the larger maf holder. what you do is determined by the year of your car (ecus changed through the years) - 2003 and younger, use an anniversary edition tune. 2002 and older, use a tune for a pre-2002 996. from your pics it looks like you have an e-cable throttle, so no issues with that or with the early 5.x ecus. your tuner will need access to a piwis or pst2 to do this work.
(c) aftermarket. there should be more, albeit expensive options here (allthough yours is a track car, so issues like poor idle, etc., may not be a big deal). it can be as simple as a resistor on the maf ground wire that lets you trim the signal going to the ecu. a piggyback ecu (simply put, acts like a programmable version of the resistor noted above). an aftermarket tune where the tuner has modified the tables that convert maf signal to airflow (harder to find than you think - not a lot of tuners can do our 7.2 ecu; may more can do the 2003- 7.8 ecu). or run alpha n - a good tuner should be able to set that up for you.

04-29-2016, 06:59 AM	#14
The Radium King Registered User Join Date: Apr 2010 Location: Canada Posts: 3,121	yessir. as has been said, without the maf the ecu uses the throttle position sensor to estimate how much fuel to provide. called alpha N tuning i recall, and is used by some race companies (gets the maf out of the system for better airflow). in our applications it is a limp mode, however, so, as stated, the ecu adds additional fuel to keep things on the rich side (rich burns fuel and kills power, lean kills engines). the guys who put the 3.4 in your car probably just kept the old tune on it - the ecu is has enough range to adapt to the increased airflow required for the bigger engine. however, note that the ecu calculates airflow by looking at the maf signal and multiplying it by the diameter of the maf holder. increase the maf holder diameter without telling the ecu and it starts doing bad math - there is more air getting to the engine than the ecu knows about and the engine runs lean. your engine will run like crap. your options are to: (a) go back to the original, smaller maf holder. (b) push an oem tune onto the car that knows the larger maf holder. what you do is determined by the year of your car (ecus changed through the years) - 2003 and younger, use an anniversary edition tune. 2002 and older, use a tune for a pre-2002 996. from your pics it looks like you have an e-cable throttle, so no issues with that or with the early 5.x ecus. your tuner will need access to a piwis or pst2 to do this work. (c) aftermarket. there should be more, albeit expensive options here (allthough yours is a track car, so issues like poor idle, etc., may not be a big deal). it can be as simple as a resistor on the maf ground wire that lets you trim the signal going to the ecu. a piggyback ecu (simply put, acts like a programmable version of the resistor noted above). an aftermarket tune where the tuner has modified the tables that convert maf signal to airflow (harder to find than you think - not a lot of tuners can do our 7.2 ecu; may more can do the 2003- 7.8 ecu). or run alpha n - a good tuner should be able to set that up for you.